Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_field_rename_2_frog_test.dart

Index: tests/compiler/dart2js_native/native_field_rename_2_frog_test.dart
diff --git a/tests/compiler/dart2js_native/native_field_rename_2_frog_test.dart b/tests/compiler/dart2js_native/native_field_rename_2_frog_test.dart
index cb093cb6ae199e414483e8aceeff4e99784be2cc..1d3cdcc1bd87eb4ae51443753a31e110e05f2aad 100644
--- a/tests/compiler/dart2js_native/native_field_rename_2_frog_test.dart
+++ b/tests/compiler/dart2js_native/native_field_rename_2_frog_test.dart
@@ -6,8 +6,7 @@
 // fields.  However, native fields keep their name.  The implication: a getter
 // for the field must be based on the field's name, not the field's jsname.
 
-import "package:expect/expect.dart";
-import 'dart:_js_helper' show Native, JSName;
+import 'native_testing.dart';
 
 abstract class I {
   int key;
@@ -47,13 +46,15 @@ X.prototype.key = function(){return 666;};
 
 makeA = function(){return new A};
 makeX = function(){return new X};
+
+self.nativeConstructor(A);
+self.nativeConstructor(X);
 """;
 
 testDynamic() {
-  var things = [makeA(), new B(), makeX()];
-  var a = things[0];
-  var b = things[1];
-  var x = things[2];
+  var a = confuse(makeA());
+  var b = confuse(new B());
+  var x = confuse(makeX());
 
   Expect.equals(111, a.key);
   Expect.equals(222, b.key);
@@ -91,6 +92,7 @@ testTyped() {
 }
 
 main() {
+  nativeTesting();
   setup();
 
   testTyped();